The present invention relates to a circuit for supplying power to an exposure lamp and, more particularly, to a AC circuit which maintains a constant lamp voltage irrespective of variations in the AC voltage supply.
Exposure lamps for purposes such as illuminating documents to be copied in a copier must be constructed so that any intensity variations are held to approximately a .+-.1%. One source of variation in lamp output which may exceed this tolerance are fluctuations and variations in the main ac supply line. In a perfect system, the main voltage wave form is a perfect sine wave having a peak amplitude equal to the .sqroot.2 times the RMS voltage. However, in practice, the amplitude may be less due to voltage drops in the supply cable, and the sine wave form may be distorted in two main ways. Firstly, the peaks of the wave may be suppressed due to saturation of transformers used in the supply network. Secondly, at a site where thyristor or triac controlled loads are in use, the form of each half wave may be reduced in a final portion of the half wave due to the increased load on the supply compared with the initial portion of the half wave. In other words, when an exposure lamp is driven by a phase controlled electronic switch such as a triac direct from an input AC source, then the shape of the AC waveform directly affects the lamp intensity.
Various techniques are known in the art to compensate for lamp intensity variations. In one technique, the output of the lamp is coupled to, and measured by, a photosensor. The photosensor output is sent via a feedback loop, to control the power output of the lamp. In a second method a smaller lamp is connected in parallel across a main lamp and the light output of the second lamp is monitored and used to control the lamp via feedback loop. A third, more commonly used procedure is to measure the voltage across the lamp using a dedicated isolation transformer with the transformer secondary connected into a feedback control system which alters the firing phase angle of the switch controlling power to the lamp. The present invention improves over the prior art by providing precise lamp illumination control of less than .+-.1% variation without the necessity for using relatively expensive photosensors or dedicated transformers. This is accomplished by circuitry which simulates the operation and the characteristics of the lamp to produce a voltage output which very closely matches the lamp voltage. This simulated voltage is fed back to a triac switch via a RMS to DC converter. A feedback controller is used to control the firing phase of the triac. More particularly, the invention relates to a circuit for supplying constant RMS voltage to a lamp from an AC voltage supply comprising a first switching element for applying a voltage V1 across a lamp upon application of power from the AC supply, sampling means synchronized with application of said AC voltage to said first switching element for generating an instantaneous RMS voltage V2 modeled to conform to said voltage V1, conversion means for converting said RMS voltage V2 into a DC voltage, and feedback controller means for comparing said converted DC voltage with a reference voltage representing an optimum lamp intensity output and for generating and applying a firing pulse to said switching element thereby controlling the phase angle of said switching element.